1. Field of the Invention
This invention relates to the art of electrical generator systems for driving piezoelectric transducers, and more particularly concerns such a system which generates driving signals over a wide band of frequencies and which employs signal sampling circuitry and feedback network loops.
2. Description of the Prior Art
Resonant operation of a piezoelectric transducer device may be defined as the frequency at which electrical to mechanical transformation takes place and may be graphically depicted by a rapidly changing phase shift with frequency. Since it is an energy exchange system, where the mechanical to electrical conversion is known to be reciprocal, abrupt changes in the load current phase presented by the piezoelectric device in motion result as energy is absorbed and reflected. Depending on whether the transducer is "squeezing" or "unsqueezing" in relation to its drive it may absorb or return electrical energy.
At frequencies below resonance for all high quality mechanically unloaded piezoelectric resonators including quartz crystals generally used for frequency control in communications equipment, the phase relationship is relatively fixed with the current leading the impressed voltage by approximately 90.degree. much the same as in a capacitor and indicative of no mechanical motion. As frequency increases and vibration begins, the phase starts to lag going through 0.degree. phase shift and finally to 90.degree. lagging. The current magnitude reaches a peak at the 0.degree. point which is generally referred to as series resonance, and goes to a minimum and nearly vanishes at 90.degree. lagging, generally referred to as parallel resonance or anti-resonance. Further increase in frequency, results in the emergence of a current, which is again leading by approximately 90.degree., very much like a capacitor. The rate of phase change with frequency lessens above the 60.degree. lagging point, culminating in a very slow change at 90.degree.. This region is sought by many conventional generators having limited stability. Analysis of the phase angle to energy delivery indicates almost full energy delivery at 0.degree. phase shift and almost full storage at 90.degree., with components of each elsewhere in the vibrating region. The resonance range, slope and smoothness of the phase curve changes with the temperature, power level, and mechanical load impressed upon the piezoelectric transducer. At the 0.degree. phase shift point, the most power can be extracted with minimum voltage stress on the piezoelectric transducer.
In a mechanically loaded piezoelectric transducer with a fixed power input such that insufficient energy is available to overcome the load and provide for operation into the lagging phase angle storage region, stoppage of the transducer occurs prior to achieving a 90.degree. lagging current. With sufficient load pressure no motion at all occurs, and the piezoelectric transducer becomes a passive capacitor throughout the entire range. When operation is at or near parallel resonance, energy is tending toward being fully reflected and high voltages are necessary to input power for conversion, thus burdening the piezoelectric transducer with dielectric breakdown and high reactive circulating current handling problems. The 90.degree. lagging, parallel resonance point is at the end of the region where rapid phase shift with frequency and energy conversion occurs. After the 90.degree. lag or parallel resonance point is reached, a discontinuity results, indicating an abrupt reversal of phase change with frequency, which mechanical load matching systems may smooth or cover to produce an apparent continuous phase sense reversal after the 90.degree. lag point, where the phase leads increasingly with frequency until the phase returns to 90.degree. leading and unchanging. This reversal of phase sense with frequency is very troublesome, preventing prior generator systems from achieving wide range, as all depend on a monotonic phase-frequency characteristic for correction of the transducer frequency.
Maintaining 0.degree. phase shift, and increasing the mechanical load by applying a force directly opposite to the motion, causes a shift upward in the vibratory frequency that affords 0.degree. until a reversal in phase change with frequency occurs. This signals sudden stoppage in transducer vibration and a return to a fixed approximately 90.degree. lead. Increasing input power with load staves off the frequency increase and vibratory stoppage. Ultimately, however, with sufficient load, a limit is reached and the vibration must be allowed to stall if transducer damage is to be avoided. Heating of the vibratory transmission system generally lowers the frequency at which the 0.degree. phase current occurs.
At the present state of the art, most generator circuits are more or less free running power oscillators only lightly influenced by the phase frequency relationship, due to difficulties in keeping the feedback from changing the frequency to points outside the resonance region, since the phase frequency relationship sense is undirectional only over a small region and may reverse suddenly under changing conditions of load, temperature, and drive level. When a phase lock loop is used, the delay associated with a resonating piezoelectric device is so great that loop stability is marginal at best and even undisturbed closed loop operation overshoots into the phase reversal region, with the consequent driving of the frequency above the frequency needed.
The prior art using the so called "self sterring" phase lock loop circuit encounters the difficulties identified and addressed herein. Circuits incorporating clamps, special amplifiers, such as disclosed in U.S. Pat. No. 4,056,761 and even microprocesseor control sequencing, such as disclosed in U.S. Pat. No. 4,577,500 to solve loop stability problems, achieve their objectives by compromising performance.